Magnetic compasses are subject to several outside influences which produce errors in the compass reading. One of these error sources is magnetic variation, which is the difference between the direction of the field lines of the earth's magnetic field and true north at any particular point in the world and which is a function of geographical position. Another source of error is magnetic deviation which is the difference between magnetic north and the actual compass reading caused by magnetic perturbations resulting from the vessel itself or equipment on the vessel. Deviation changes as a function of the heading of the vessel.
Variation is always given on navigation charts and may be compensated for by correcting the compass reading by a constant correction factor for a particular geographical area, and which factor changes relatively infrequently. Deviation-correction, however, is much more tedious since deviation is a function of vessel heading, and many different correction factors must be provided for the different headings. Deviation is commonly corrected by reference to a chart containing deviation-correction factors for different headings, of the particular vessel, such as for every 10 degrees. Since the heading may change frequently, correcting for deviation can become burdensome. Additionally, the calibration procedure by which a deviation chart is made is tedious and time consuming.